380. Insert Delete GetRandom O(1)
Design a data structure that supports all following operations in average O(1) time.
insert(val): Inserts an item val to the set if not already present.remove(val): Removes an item val from the set if present.getRandom: Returns a random element from current set of elements. Each element must have the same probability of being returned.
设计一个数据结构，使得插入、删除，随机获取一个元素的时间复杂度都是O(1)
1 class RandomizedSet { 2 3 ArrayList<Integer> nums; //存放具体的值 4 HashMap<Integer, Integer> locs; //标记每个值在nums中的位置 5 java.util.Random rand = new java.util.Random(); 6 /** Initialize your data structure here. */ 7 public RandomizedSet() { 8 nums = new ArrayList<Integer>(); 9 locs = new HashMap<Integer, Integer>(); 10 } 11 12 /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */ 13 public boolean insert(int val) { 14 boolean contain = locs.containsKey(val); 15 if ( contain ) return false; 16 locs.put( val, nums.size()); 17 nums.add(val); 18 return true; 19 } 20 21 /** Removes a value from the set. Returns true if the set contained the specified element. */ 22 public boolean remove(int val) { 23 boolean contain = locs.containsKey(val); 24 if ( ! contain ) return false; 25 int loc = locs.get(val); 26 if (loc < nums.size() - 1 ) { // not the last one than swap the last one with this val 27 int lastone = nums.get(nums.size() - 1 ); 28 nums.set( loc , lastone );//将最后一个位置的元素拷贝到本位置，达到覆盖被删除元素的目的 29 locs.put(lastone, loc); //因为map中最后一个位置的元素被移走了，所以要同步修改mamp中的位置索引 30 } 31 locs.remove(val); //删除map中要被删除元素 32 nums.remove(nums.size() - 1);//删除list中最后一个元素 33 return true; 34 } 35 36 /** Get a random element from the set. */ 37 public int getRandom() { 38 return nums.get( rand.nextInt(nums.size()) ); 39 } 40 }
